A frequency mixer is one of the basic elements in many communication devices such as, for instance, mobile phones. Frequency mixers are used in frequency-to-frequency conversion and are critical components in modern radiofrequency (RF) systems. Generally, a frequency mixer converts RF power at one frequency into power at another, shifted frequency. This can make subsequent signal processing easier and inexpensive.
A fundamental reason for frequency conversion is to allow amplification of the received signal at a frequency other than the applied RF, or audio, frequency. For instance, a receiver may require as much as 140 decibels (dB) gain. However, it might not be possible to put more than 40 dB gain into the RF section of the receiver without risking instability and potential oscillations. Similarly, the gain of the audio portion of the receiver may be limited to 60 dB because of parasitic feedback paths and microphonics. The additional gain needed for a sensitive receiver is normally achieved in a mixed frequency (MF) section of the receiver.
FIG. 1 illustrates a schematic representation of an ideal mixer, which multiplies two input signals: RF input and LO input. RF input corresponds to the radiofrequency (RF) input to the mixer at a first frequency f1 while LO input corresponds to the Local Oscillator input frequency f2. The mixer multiplies the two input frequencies (f1, f2) and produces a mixed frequency (MF). Assuming that the two input frequencies (f1, f2) are sinusoids, the ideal mixer illustrated in FIG. 1 produces two output frequencies which include the sum (f1+f2) and difference (f1−f2) of the frequencies.
Currently, RF mixers are built using electronic devices such as diodes, field effect transistors (FETs), and bipolar transistors, which have non-linear I-V characteristics to provide the frequency conversion mechanism. Unfortunately, there are some downsides to current electronic-based mixer devices. First, these devices utilize power because of the electric current-based non-linear elements. Power consumption is increasingly important as commercial devices are made ever smaller and require longer operational use times. In addition, electronic devices may not be compatible with high frequency operations (e.g., those in the GHz or THz range) because of internal delays caused by the electronic components.
There thus is a need for alternative frequency conversion devices (e.g., RF mixers) to conventional electronic-based devices. Such a device should be scalable which can be made in a very small size using conventional semiconductor manufacturing processes. The device should be able to operate at high frequencies, including frequencies in the GHz and THz range. Power consumption should also be reduced to the extent possible.